Effects of chemical bonding on heat transport across interfaces

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1 Effects of chemical bonding on heat transport across interfaces Mark D. Losego 1,2*, Martha E. Grady 3, Nancy R. Sottos 1,2,3, David G. Cahill 1,3, and Paul V. Braun 1,2 1 Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL USA 2 Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana- Champaign, Urbana, IL USA 3 Department of Mechanical Science and Engineering, University of Illinois at Urbana- Champaign, Urbana, IL USA I. Supplementary Methods I. A. Characterization Tools and Methods SAM thickness was characterized using a single-wavelength (633 nm) ellipsometer (Gaertner L116C). Because of the similarity in the refractive index of quartz and alkyl SAMs, ellipsometry could not be conducted directly on quartz substrates. Thus, for ellipsometric measurements, SAMs were concurrently deposited on silicon substrates having a native oxide in the same reaction vessel. A refractive index of 1.5 was used for the SAM and the native oxide was measured before SAM deposition (native oxide thickness is ~22 Å). Reported contact angles are for static measurements of water droplets on the SAM surface. Atomic force microscopy (AFM) was conducted in tapping mode using an Asylum Research MFP-3D system. X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis ULTRA photoelectron spectrometer system with an Al K-α x-ray source ( ev). A Phillips X Pert MRD system is used to conduct x-ray reflectivity (XRR) measurements of the gold film s thickness. NATURE MATERIALS 1

2 I. B. Preparation of Self-Assembled Monolayers Self-assembled monolayers (SAMs) were deposited on z-cut quartz wafers (MTI Corporation, Richmond, CA) for both its well-known surface attachment chemistry (hydroxylated surface chemistry) and its high thermal conductivity, which is needed as a heat sink for the thermal measurements. Single crystal oxide substrates also offer the advantage of not having a native oxide layer, which significantly contributes to the interfacial thermal conductance and complicates accurate modeling of the TDTR data 1,9. The quartz substrates were cleaned in piranha solution (3 H 2 SO 4 : 1 H 2 O 2 by volume) at 65 C for 60 min (WARNING: piranha is a strong oxidant and must be handled carefully). The quartz pieces were then rinsed with copious amounts of water, dried under a stream of nitrogen, and further dried in an oven at 120 C for 30 min in air. This procedure was done to leave only a monolayer of water on the surface as the activator for silane attachment. Silicon pieces with a native oxide layer were prepared in the same manner and SAMs were prepared on both quartz and silicon surfaces in the same reaction vessel. The silicon substrates were then used for ellipsometry measurements. Optimization of SAM deposition conditions is described in Sections II.A.1 and II.A.2. General procedures involve immersing the substrates in a toluene solution (ACS certified, Fisher Scientific) of 10 mm silane plus 15 mm triethylamine for 24 hours in a sealed container on the benchtop. Silanes used in this experiment are listed in Table S1. Table S1: List of chemicals used to produce self-assembled monolayers. Chemical Formula Notation Purchased From Sigma Aldrich Dodecyltriethoxysilane CH 3 (CH 2 ) 11 Si(OC 2 H 5 ) 3 CH 3 -C 11 -Si St. Louis, MO 11-mercaptoundecyltrimethoxysilane 2 ) 11 Si(OCH 3 ) 3 SH-C 11 -Si Gelest, Inc SH(CH Morrisville, PA 11-aminoundecyltriethoxysilane 2 (CH 2 ) 11 Si(OC 2 H 5 ) 3 NH 2 -C 11 -Si Gelest, Inc NH Morrisville, PA 11-bromoundecyltrimethoxysilane 2 ) 11 Si(OCH 3 ) 3 Br-C 11 -Si Gelest, Inc Br(CH Morrisville, PA Dodecyl-dimethylmonochlorosilane 3 (CH 2 ) 11 SiCl(CH 3 ) 2 CH 3 -C 11 -Si(CH 3 ) 2 - Sigma-Aldrich CH St. Louis, MO 2 NATURE MATERIALS

3 I. C. Transfer-Printing of Gold Films Transfer-printing of gold films to the SAM modified quartz substrates followed the same procedures as Meitl et al. 14 and is depicted in Figure 1 (c-f) of the text. Silicon substrates with thermally grown oxide layers (~75 nm) were cleaned in a piranha solution, rinsed with copious amounts of water, and dried under a stream of nitrogen. Gold films of nominally 75 nm thickness were deposited by e-beam evaporation (Temescal) onto the SiO 2 /Si substrates. Poor adhesion at the Au/SiO 2 interface makes these oxide surfaces ideal donor substrates for the transfer-printing process. A thin (few microns) layer of PVA (87-89% hydrolyzed, MW ~ 20,000 g/mol, Sigma-Aldrich, 10 wt% dissolved in H 2 O) is cast onto the Au surface to impart mechanical stability during the transfer process. The PVA layer is dried at 85 C for 5 min. A block (approx. 2 cm x 2 cm x 0.7 cm thick) of polydimethylsiloxane (PDMS, Sylgard 184 Silicon Elastomer, Dow-Corning) is used to transfer the film. The PDMS block is manually pressed onto the donor substrate such that it completely wets the PVA/Au surface. It is then rapidly peeled, causing delamination and resulting in the PDMS stamp being inked with the gold film. The film is laminated to the receiving, pre-functionalized quartz substrate and heated on a hotplate at 115 C. After 90 s of manual pressure, the PDMS stamp is slowly peeled from the surface leaving the PVA/Au film bonded to the quartz substrate. The film is kept on the hotplate for another 90 s to finish bonding. The PVA layer is then rinsed away under flowing water. Because PDMS-Au adhesion is a function of peel rate, Au layers can be transfer-printed to any surface chemistry regardless of adhesion strength (Fig. S1). This permits the formation of control structures having a Au/SiO 2 interface as well as weakly bonded systems like Au/CH 3 - C 11 -Si Qz. NATURE MATERIALS 3

4 Fig. S1: Photographs of gold layers transfer printed to different surface chemistries including (A) quartz functionalized with 11-mercapto-undecyltrimethoxysilane (B) quartz functionalized with dodecyltriethoxysilane, and (C) a bare quartz surface cleaned using a piranha treatment. II. Supplementary Discussion II. A. SAM Formation II. A.1 Optimization of SAM Deposition Conditions for Dense Monolayer Formation Monolayer deposition was carried out in solution. Experimentation with the SAM deposition procedures revealed that anhydrous solvents and preparation in an inert atmosphere did not improve monolayer formation for alkoxysilanes (Figure S2). However, the use of an organic base to catalyze hydrolysis of the alkoxysilane was found to improve the monolayer density. As Figure S2 shows, the use of triethylamine leads to monolayers of significantly greater thickness approaching that of a fully extended alkyl chain consistent with a dense SAM. Presumably, the base catalyst increases reaction rate at the hydrated surface causing silanes to attach to the surface before polymerization reactions occur in the solution. Standard solution preparation consisted of 10 mm silane plus 15 mm triethylamine (Et 3 N, Reagent Grade, Fisher Scientific) in 20 ml of toluene(acs certified, Fisher Scientific). Substrates were immersed in solution within a sealed container for 24 hours. Immersion periods longer than 24 hours did not significantly increase the monolayer thickness nor the contact angle. 4 NATURE MATERIALS

5 Fig. S2: Ellipsometric and water contact angle investigations of SAMs prepared in ambient atmosphere with reagent grade solvents versus in an argon glovebox with anhydrous solvents on silicon substrates with and without an amine catalyst. (a) ellipsometric thickness and (c) water contact angle of SAMs prepared without the amine catalyst in solution; (b) ellipsometric thickness and (d) water contact angle of SAMs prepared with the amine catalyst in solution. Error bars are 95% confidence intervals created from > 5 measurements. II. A.2 Avoiding Disulfide Formation Because dissolved oxygen is sufficient to oxidize thiols into disulfides, we add the reducing reagent dithiothreitol (DTT, Sigma-Aldrich) to limit this reaction pathway during preparation of 11-mercapto-undecyltrimethoxysilane (SH-C 11 -Si) monolayers. Figure S3 compares SH-C 11 -Si SAMs formed with and without DTT. Without the reducing agent, ellipsometry reveals a much thicker organic layer consistent with polymerization due to disulfide and subsequent silane reactions. When DTT is incorporated in the silane solution, a thickness of ~13 Å is achieved, which is consistent with the thickness expected for a SH-C 11 -Si molecule in NATURE MATERIALS 5

6 the all trans conformation. AFM images of the SAM surface shown in Figure S4 also confirm reaction agglomerates on the surface when SH-C 11 -Si SAMs are prepared without DTT (Figure S4a) but a smooth monolayer when prepared with DTT (Figure S4b). Ellipsometric measurements of a SAM thickness consistent with an all trans conformation combined with smooth AFM scans allow us to conclude that our SAMs are densely packed. This combination of attributes was also confirmed for SAMs prepared having mixtures of thiol and methyl termination. The only SAMs to show a smaller thickness by ellipsometry than expected were the dodecyl dimethylmonochlorosilanes, indicating a poor packing density. Figure S3: Thickness of SH-C 11 -Si SAMs prepared in toluene without the DTT reducing agent (No DTT), after rinsing with a 0.1 M aqueous solution of DTT for 10 min (No DTT + Rinse) and with DTT dissolved in the SAM deposition solution (DTT). Error bars are 95% confidence intervals created from > 8 measurements. Figure S4: AFM topography images of SH-C 11 -Si SAMs prepared on quartz (a) without DTT and (b) with DTT. 6 NATURE MATERIALS

7 II. A.3 Characterization of Thiol Termination in Mixed Monolayer SAMs An XPS analysis of the surface chemistry was conducted for SAMs prepared using the mixed monolayer chemistry (SH-C 11 -Si + CH 3 -C 11 -Si ). To avoid charging, XPS was conducted on SAMs prepared on silicon substrates that were simultaneously prepared with the SAMs on quartz used for TDTR measurements. The spectra are adjusted to the C 1s photoelectron line, which we position at ev. The S 2s photoelectron lines are shown in Figure S5 for varying concentrations of SH-C 11 -Si in the precursor solution. The S 2s signal decays as expected with decreasing solution concentration of SH-C 11 -Si. Because these partial monolayers of thiols give a fairly weak photoelectron emission signal, quantification is difficult. If we assume the 100% thiol surface is a full monolayer, we can estimate concentrations based on the area under the peak fits to the data as ~43% for the 75% thiol solution, ~22% for the 50% thiol solution and ~17% for the 25% thiol solution. Figure S5: XPS data collected from mixed monolayers of SH-C 11 -Si + CH 3 -C 11 -Si showing the S 2s photoelectron line. NATURE MATERIALS 7

8 II. B.1 Time-Domain Thermoreflectance Measurements Time-domain thermoreflectance (TDTR) is used to measure the thermal conductance of the Au/SAM/Qz structures. TDTR, described previously 4, is a modulated optical pump-probe technique that uses a pulsed Ti:sapphire laser to monitor temperature decay by thermoreflectance with picosecond resolution. The pump pulses are modulated at 9.8 MHz with an electro-optic modulator. The reflectance of the probe beam is detected at the pump s modulation frequency using a lock-in amplifier. The ratio of the in-phase to out-of-phase signal (V in /V out ) is then fit to a thermal diffusion model 17. If the thickness, heat capacity, and thermal conductivity are accurately known for the substrate and metal transducer, then a fit of the model to the data provides the thermal conductance of the interface. For our system, Au is acting as the transducer (converting the laser pump pulse into thermal energy) and as the thermoreflectance sensor (to monitor surface temperature). Since acoustic waves travel at a few nanometers per picosecond, the depth resolution of TDTR is approximately the same as the SAM thickness (~1 nm). Thus, we treat the SAM layer as an abrupt interface in the thermal model and its conductance is assumed to be the interfacial thermal conductance (G) at the Au/Qz interface. Thermoreflectance of gold is relatively poor at 787 nm 31,32, which is the optimized wavelength for the optics of the TDTR setup at the Frederick Seitz Materials Research Laboratory at the University of Illinois. To enhance the signal, a larger than normal pump intensity (80 mw) is used to increase the temperature rise. At typical experimental conditions (80 mw laser intensity, 7 µm Gaussian FWHM laser spot size, 11 W/mK thermal conductivity for quartz, and reflectivity of 0.97 for Au at 787 nm), we estimate the single-pulse temperature rise to be ~1 C and the steady state heat rise to be ~8 C. 17 For a standard Al film transducer, a similar 1 C single-pulse temperature rise can be achieved using only a ~20 mw pump pulse, 8 NATURE MATERIALS

9 leading to only a 1 2 C steady state temperature rise. The lower absorptivity of Au at 787 nm and the lower thermoreflectance signal requires the use of this larger pump pulse intensity. However, the quartz substrate can sufficiently dissipate this heat flux. Steady state temperature rises of < 10 C at room temperature are acceptable for TDTR experiments. The laser spot for TDTR has a 1/e 2 diameter of ~7 µm when a 10x focusing objective is used. This integrated optical objective allows us to image the sample and choose uniform areas of this size to probe on the sample surface. An optical image of a typical transfer-printed surface (shown in Fig. S6) demonstrates that uniform areas of ~7 µm diameter are abundant across the surface. Data was collected from at least 3 points across a sample, and the averaged TDTR signal was used for modeling. Variation in the TDTR signal between points on the same sample was typically < 5%. Figure S6: Optical microscopy images showing the surface of a representative transfer printed gold film. Lower magnification image shows macroscopic defects that are easily avoidable during TDTR measurements, which use a 7 µm laser spot size. NATURE MATERIALS 9

10 To extract values for G across the Au/Qz interface, the thickness, thermal conductivity, and heat capacity for all other layers must be known. The heat capacity for Au and quartz is obtained from tabulated values. The Au layer thickness is measured after transfer-printing using x-ray reflectivity (XRR). Thermal conductivity for Au is obtained by measuring the in-plane electrical conductivity and then computing thermal conductivity from the Wiedemann-Franz law. We use the first 10 nm of the Au film as an absorption layer. This layer is modeled as 1 nm thick and having 10x higher thermal conductivity and heat capacity than the Au film 17. We summarize the modeling parameters in Table S2. Table S2: Parameters used in thermal conductivity model to fit TDTR data. Thermal Layer Thickness Heat Capacity (C Conductivity (Λ) p ) Absorption Layer 1 nm 10 x Λ Al 10 x C p, Al (24.9 J/cm 3 K) Gold Measure by XRR, subtract 10 nm Measure elec. cond. by 4-point probe and calculate therm. cond. from W-F Law 2.49 J/cm 3 K SAM/Interface 1 nm Fitting Parameter 0.1 J/cm 3 K Quartz 1 cm Fitting Parameter always falls between 10.5 and 12.5 W/mK 1.96 J/cm 3 K As demonstrated in Fig. 2a of the text, changes in the interfacial thermal conductance change the curvature of the V in /V out signal. Most parameters incorporated in the thermal diffusion model only shift the magnitude of the V in /V out signal and have little effect on the curvature. Thus, we use this change in the shape of the V in /V out signal as a sensitive measure of G. We describe this process in Fig. S7. Here, we assume that errors in gold thickness (either from the XRR measurements or our assumptions about the absorption layer thickness) cause variability in the magnitude of V in /V out. The solid line is a simulation using Au thickness = 75 nm, Λ Qz = 12 W/mK, and G = 40 MW/m 2 K. The dotted line represents an error in Au thickness of +3 nm. (We expect XRR to be accurate to within +/-1 nm, but modeling of the Au absorption layer is not 10 NATURE MATERIALS

11 well understood.) The red circles and blue squares show how simulations for a 78 nm Au layer (75 nm + 3 nm) can be adjusted by changing Λ Qz or G to give approximately the same fit as the 75 nm layer at short delay times (< 300 ps). By adjusting Λ Qz only (red circles), we end up with a fit having about the same curvature as the simulation for a 75 nm Au layer. By adjusting G only (blue squares), the curvature changes significantly. Thus, we believe fitting the curvature of the V in /V out signal is a more accurate method for determining G than simply fixing Λ Qz to a constant value. In general, when we fit G to match the experimental curvature and Λ Qz to adjust the magnitude of the signal, we obtain values for Λ Qz ranging from 10.5 to 12.5 W/mK, which are consistent with the accepted value of ~12 W/mK for quartz along the c-axis 33. Figure S7: Simulations of the V in /V out signal for TDTR measurements similar to the experimental system (solid line). Example of the change in the V in /V out signal when an error of +3 nm is introduced to the gold thickness (dotted line). Red circle and blue squares represent possible corrections for this error in thickness by respectively adjusting the thermal conductivity of the quartz substrate and the interfacial thermal conductance (G). NATURE MATERIALS 11

12 Figure S8: Example of how the error in G was determined through fitting of the thermal model. For figures in the text, the error bars reported for G are based on noise in the collected V in /V out signal. Thermal simulations with varying G that create curvatures that encompass all of the noise in the data at long delay times are used to define the error as demonstrated in Figure S8. II. B.2 Pico-second Acoustic Measurements The same pump-probe setup was also used to collect the picosecond acoustic data presented in the inset of Figure 2a. The pump pulse excites longitudinal normal modes in the gold thin film. Monitoring how the fundamental normal mode damps with time reveals information about the interfacial stiffness. 20,21 Because the piezoreflectance (reflectivity as a function of strain) of Au is weak at these wavelengths, a modified pump-probe geometry was used to increase the signal-to-noise. The pump and probe beam locations on the sample were spatially separated such that the probe only partially overlapped the heated region. Thus, only a portion of the probe lies incident on the strain field causing a spatial deflection of the beam. An aperture was used to partially block the reflected probe upon deflection leading to periodic intensity oscillations. Over one oscillation period, the strain wave travels to the Au/SAM interface, gets reflected, and returns to the Au surface. The intensity of these oscillations decay with time because some portion of the acoustic wave is transmitted across the interface rather 12 NATURE MATERIALS

13 than be reflected. Thus, the decay rate of the oscillations can be related to how well the interface can transmit acoustic energy, which can be related to interfacial bond strength. 20 To analyze the collected data, the offset of the V in /V out data was removed by subtracting the saturation value at long delay times. Then the data was fit to the damped harmonic oscillator equation plus an exponentially decaying background signal using a least squares regression routine. The fitting equation had the following functional form: (1) where t is time, Γ is the oscillator s damping coefficient, T is the oscillation period, δ is the phase angle offset, τ is the background decay constant, and A and B are intensity scaling parameters. The reflectivity (r) is calculated from 20 : (2) II. C. Laser-Induced Spallation Experiments Figure S9 shows the test structures prepared for laser spallation testing. Fused silica substrates were functionalized with either CH 3 -C 11 -Si or SH-C 11 -Si SAMs. A 185 nm Au film was transfer printed on to the functionalized surface. On the backside of the sample, a 400 nm thick aluminum (Al) absorbing layer was deposited by e-beam evaporation followed by spin casting a 1 µm sodium silicate (waterglass) confining layer. Figure S9: Depiction of layered structure used for laser spallation measurements NATURE MATERIALS 13

14 Figure S10: Schematic of laser spallation test set up. Adhesion measurements were made with a laser-induced spallation setup that has been described previously 34 and is shown schematically in Figure S10. A rapid, high-amplitude acoustic wave was initiated by the impingement of an Nd:YAG pulsed laser (New Wave Tempest) on the Al energy absorbing layer on the back surface of the specimen. Because the sodium silicate layer confines expansion of the Al layer, a compressive acoustic wave is generated in and propagates through the fused silica substrate. Upon reflection from the substrate/au interface, the wave loads the thin film-substrate interface in tension. A Michelson interferometer is used to measure the displacement of the free surface. A biased silicon photodetector (Electro-Optics Technology ET-2030) connected to a high-rate oscilloscope (LeCroy LC584 A) records the temporal interference pattern as a voltage trace, V(t), given by, (3) where V max and V min are the voltage maximum and minimum respectively of each interference fringe and t is time. The interference fringe number, n(t), is unwrapped and then converted to displacement (u(t)) using, 14 NATURE MATERIALS

15 (4) where the wavelength of the interferometric laser, λ 0, is 532 nm. The substrate stress σ sub is then calculated from the displacement using Eq. (5) and the substrate material properties are listed in Table S3. Table S3: Material properties used to calculate substrate stress (5) In order to determine the substrate and interface stresses for a given laser fluence (energy per area), we carried out a set of calibration experiments with highly reflective thin Al films (200 nm) that were electron-beam deposited on identical (500 µm)fused silica/(0.4 µm)al/(1 µm) waterglass substrates. A calibration set is necessary because the Au/SAM/SiO 2 films partially fail at laser fluences of interest, invalidating the in situ interferometry measurements. Thus, an Al film, which has a higher interface strength and larger reflectivity, is used to calibrate the stress wave magnitude as a function of laser fluence. A laser spot size of diameter 2 mm is chosen and kept constant for these experiments. The laser energy is incremented, and the interference pattern captured at a sampling rate of 8 GS/s. The displacement of the free surface and substrate stress are calculated for each increment in laser energy. The substrate stress pulse measured during calibration provides input for a 1D finite element analysis that calculates the interface stress for a 185 nm Au film. The peak interface stress for each substrate stress pulse is averaged for the corresponding laser fluence. From this data, a relationship between the laser fluence and the interface stress is determined, as shown in Figure S11. NATURE MATERIALS 15

16 Figure S11: Data for Al films used to calibrate interface stress vs. laser fluence in experimental structures. Error bars are standard error calculated from multiple measurements. After testing over a range of fluence values, optical microscopy is used to investigate the loaded regions of the Au/SAM/SiO 2 structures. Unlike most previous studies using physical vapor deposited (PVD) films, these transfer-printed Au layers show non-abrupt failure. For typical PVD deposited films, a critical laser fluence value exists at which all films will show some delamination. In these transfer printed films, there is a range of fluences where only a fraction of the films will show delamination. Figure S12 plots this behavior showing the fraction of films at each laser fluence that exhibit delamination. An example of the non-abrupt behavior is the Au/SH-C 11 -Si sample at 10.4 mj/mm 2 where only 40% of the 10 spots tested showed delamination. We believe this non-abrupt behavior is due to the inherent macroscopic defects in these transfer-printed layers as described in Figure S6. However, as Figure S12 demonstrates, both the onset of delamination and full delamination occur at higher laser fluences for the Au/SH-C 11 -Si as compared to the Au/CH 3 -C 11 -Si structures, confirming the stronger bonding at the thiol-au interface. 16 NATURE MATERIALS

17 Figure S12: Plot of the fraction of spots tested that showed delamination for both Au/ SH-C 11 -Si and Au/CH 3 -C 11 -Si interfaces as a function of the laser fluence. The calibrated interface stresses are also shown on the top axis. Numbers near each data point represent the number of spots tested. For this work we define the critical laser fluence as the lowest fluence value that initiates delamination 100% of the time. The interface strength is then determined from the corresponding average interface stress value in the calibrations conducted at this critical laser fluence. For structures with CH 3 -C 11 -Si molecules at the interface, this critical laser fluence is only 12.6 mj/mm 2, which corresponds to an interface stress of 24.2 MPa. For structures with SH-C 11 -Si at the interface, this critical laser fluence is 19.0 mj/mm 2, which corresponds to an interface stress of 60 MPa. References Ujihara, K. Reflectivity of Metals at High-Temperatures. J. Appl. Phys. 43, 2376-& (1972). Wang, Y. X., Park, J. Y., Koh, Y. K. & Cahill, D. G. Thermoreflectance of metal transducers for time-domain thermoreflectance. J. Appl. Phys. 108 (2010). CRC Handbook of Chemistry and Physics. 74th edn, (CRC Press, 1994). Wang, J. L., Weaver, R. L. & Sottos, N. R. Laser-induced decompression shock development in fused silica. J. Appl. Phys. 93, (2003). NATURE MATERIALS 17